Proteins: Amino acids and structural levels of proteins |
Biotechnology
### Proteins: Amino Acids and Structural Levels
**Amino Acids: The Building Blocks**
Proteins are linear polymers composed of 20 standard L-α-amino acids. Each amino acid consists of a central carbon (Cα) bonded to an amino group (-NH₃⁺), a carboxyl group (-COO⁻), a hydrogen atom, and a variable side chain (R-group). The R-group determines chemical properties: nonpolar/hydrophobic (e.g., leucine, valine), polar uncharged (e.g., serine, glutamine), positively charged (lysine, arginine, histidine), or negatively charged (aspartate, glutamate). Amino acids link via **peptide bonds**—amide linkages formed by dehydration between the carboxyl group of one amino acid and the amino group of the next. This generates a polypeptide backbone with alternating N-Cα-C atoms, and side chains projecting outward.
**Levels of Protein Structure**
1. **Primary Structure:** The linear sequence of amino acids from N-terminus to C-terminus. This sequence encodes all higher-order structural information (Anfinsen's dogma). A single amino acid substitution (e.g., sickle-cell anemia: Glu6Val in hemoglobin) can cause dramatic functional consequences.
2. **Secondary Structure:** Local folding patterns stabilized primarily by hydrogen bonds between backbone amide and carbonyl groups. Common motifs include:
- **α-helix:** Right-handed coil with 3.6 residues per turn, stabilized by i → i+4 hydrogen bonds.
- **β-sheet:** Extended strands arranged laterally, either parallel or antiparallel, stabilized by inter-strand hydrogen bonds.
- **Turns and loops:** Connect secondary elements; β-turns reverse polypeptide direction.
3. **Tertiary Structure:** The three-dimensional global fold of a single polypeptide chain, stabilized by hydrophobic effect (core burial), hydrogen bonds, disulfide bridges (covalent between cysteine residues), electrostatic interactions (salt bridges), and van der Waals forces. Domains are independently folding structural units.
4. **Quaternary Structure:** The spatial arrangement of multiple polypeptide subunits (identical or different) into a functional multimeric protein. Stabilizing interactions are similar to tertiary. Examples include hemoglobin (α₂β₂ tetramer) and DNA polymerase.
Structural levels are hierarchical but interdependent; tertiary packing influences secondary element placement. Understanding these levels is fundamental for predicting function, designing mutants, and interpreting disease-causing mutations.